Paramagnetic Scattering of Neutrons by an Iron-Nickel Alloy

Paramagnetic neutron scattering has been observed from an alloy of 65% Fe-35% Ni at temperatures up to about twice the Curie temperature (Tc=490°K). A cold-neutron technique was employed with energy analysis of the scattered beam so that all scattering processes with energy transfers below about 0.1 eV could be observed. The phonon scattering, which is of comparable intensity to the paramagnetic scattering, has been subtracted by two independent techniques that give results substantially in agreement with one another. The paramagnetic scattering integrated over energy transfer can be associated with a mean value of S(S+1) for the scattering atoms giving an apparent iron atom moment of 1.4±0.3 μB. The scattering has a half-height of 0.050±0.015 eV for wave vectors which are not near the center of a zone. This figure is an order of magnitude greater than has been predicted for a simple band model which ignores correlation effects and intra-atomic exchange. The existence of so much paramagnetic scattering at such small energy transfers indicates that atomic moments in the paramagnetic phase are defined for a relatively long period of time.